Attenuating and selecting circuts



Dec, 5, 1933.

R. E. SMITH El AL ATTENUATING AND SELECTING CIRCUITS Filed Jan.

F F F f J FA FREQUENCY FIG. I

, V 5 7 CE I 4 5 4 4 INVENTORS ROTHWELL E.SMITH LEO A. KELLEY ATTORNEY Patented Dec. 5 1933 ATTENUATING AND SELECTING CIRCUITS Rothwell E. Smith, Westfield, N. J., and Leo A. Kelley, Jackson Heights, N. Y., 'assignors to In tel-national Communications Laboratories, Inc.,' New York, N. Y., a corporation of New York Application January 23, 1931. Serial No. 510,676 a 3 Claims. (01. 17844) This invention relates to electrical signalling systems and more particularly to apparatus for selecting and attenuating electrical energy of .various frequencies transmitted over a common line. i i

It is well known that the. frequencies of separate channels in traversing a line may arrive at the receiving point at unequal energy levels. When one considers that receiving apparatus is generally so designed that it operates most satisfactorily at some particular energy level, which in practice is not necessarily ob-' tained under ordinary working conditions, one can readily perceive that suitable means for adjusting the energy level as required in the difierent receiving channels should be provided; If the energy level is right for one channel it may be too high or too low for other channels, and therefore suitable regulating means should be included.

It is one of the objects of this invention to provide a new and improved arrangement for attenuation purposes with the necessary regulating elements. incorporated in the individual re ceiving circuits, and capable of adjustment as the particular frequency and other conditions of each channel may demand. g a

Another object is to provide means by which the operator at the receiving terminal may easily obtain the most eflicient signalling voltage impressed on the demodulator.

A further object is to provide means for controlling the energy input to the receiving equipment during actual working periods.

Still another object isto provide an attenuating device whereby attenuation is obtained in a particular channel independently of other receiving channels.

Additional objects and advantages of this invention will be apparent from the following description, taken with the accompanying drawing, wherein:

Figure 1 is a graph showing the relation between transmission loss and frequency in several typical receiving channels having different carrier frequencies;

Figure 2 shows anumber of receiving channels of a carrier current signalling system employing an attenuator for accomplishing the results above described.

The same numerals identify the same parts throughout.

In Figure 1 curves A, B and C show relative transmission characteristics of electrical energy coming over a typical line and delivered to a number of receiving channels; for example, three arranged to select carrier frequencies-F1, F2 and F3. The relative levels of the energy received by the various channels are indicated by the points f1, f2 and 13 upon the line XY. Assume that the line XY represents the levels or intensity of incoming signal energy most suitable for the aforesaid three channels of the receiving apparatus. It will be noted that the energy received in the channel Working on frequency F1 is not sufiiciently attenuated; channel working on the .next higher frequency F2 by acoincidence is of the right amount; and channel-working on the frequency F3 is attenuated too much. By means of the appliance herein described, the channel working on frequency F1 whose characteristic curve is shown by A, can be afiected so that this curve is shifted to the position A. This is accomplished byincreasing the transmission loss in this channel, as by adding attenuation with the attenuator unit, to be presently described, thus moving FA on the line XY to the position FA on the line X'Y. This .is accomplished without changing the shape of the curve A.

In the case of the channel working on frequency F2, in connection with which characteristic curve B and the desired curve B are identical, no adjustment is required.

The channel working on'frequency F3 however, for which the curve C is the characteristic, is affected so that-its curve is shifted downward and FC now falls at FC' on line XY'; butthe curve C still has the same shape as the curve C; This result is accomplished by reducing the effect of the attenuator, which is assumed to have some suitable initial adjustment; Thus it on to the receiving apparatus is caused to be of the same amount in all the receiving channels; and this amount is calculated as most suit able for eflicient operation;

Figure 2 shows a number of receiving channels of a carrier signalling system employing an attenuator device in each channel, for securing the results described above. Three channels 1, 2 and 3 are associated with main line ML. Since all of the channels are alike, itwill be sufficient to describe in detail the component parts on one channel, namely 1. This channel contains coupled-tuned circuits, land 5 connected on the input side in parallel with the input circuits of the other channels. The terminals of the tuned circuit-5 are connected across the free terminals of X and Z of a continuously adjustable resistance attenuator, consisting of continuously adjustable resistances X, Y and Z. The terminals of Y and Z are connected through similar tuned circuits 6 and 7 to the receiving device. Such a resistance network, as is well known in the art, can be so constructed that, for various values of resistances X, Y and Z, various transmission losses can be adjusted at will in the circuit where it is inserted. Furthermore, it can be so designed that it does not alter substantially the impedance relation, for the range of frequencies assigned to the particular channel, between the tuned input circuits 4 and 5 and the tuned output circuits 6 and '7 of the attenuator. This property is essential in order that the overall characteristic curve of any receiving channel will not be changed in shape when the attenuator is adjusted to different values and is secured by designing the input impedance of the attenuator to equal the output impedance of the tuned circuits, 4 and 5;,and the output impedance of the attenuator to equalthe input impedance of the tuned circuits 6 and 7. By impedance of the circuits 4 and 5 or 6 and 7 is meant the impedance of each pair of circuits as a unit. When these conditions are fulfilled adjustments of the attenuator result merely in a vertical shifting of the characteristic curve, without change of shape.

The circuits may of course be balanced by inserting condensers and resistances .in both sides of the various receiving channels.

It is to be understood that the coupled tuned circuits referred to above are so designed that certain practical advantages result; such as, economy of construction and uniform impedance in the various channels. The latter feature ob vicusly permits the design of an attenuator unit which may be used interchangeably in all of the .channels. The economy. of construction is secured mainly by designing the tuned selecting circuits to permit the use of an inter-changeable coil for all of the inductances used in all the various channels. When this method of design is employed the selecting circuit units for the various channels are of identical construction and differ for diiferent channels only in the size or" thecondensers. and the amount of coupling between the pairs of inductance coils for which adjustments are easily provided in theselecting circuit units. 7

It can be seen that, with this invention, we

can obtain a device which affords signalling currents of uniform quality in circuits having uni form impedance characteristics. Furthermore, we are enabled to secure selecting and attenuating units which, in a signalling system having a piurality of channels, may be used interchangeably in all the various channels in the manner described above. Although we prefer to associate the attenuator units with tuned circuits of this character, we of course, do not wish to be limited to devices of this precise nature in the practice of our invention.

a carrier current signalling system inequalities in the transmission characteristics arising from various causes. It is readily apparent that the inequality in energy levels at the various channel frequencies can be corrected, provided the device is designed to take care of the maximum variation.

What is claimed is:

1. In a carrier telegraph system, a line, signal receiving apparatus, an attenuation device for attenuating without distortion signals received over said line, said device having an input circuit tuned to select a predetermined frequency band and coupled to said line and an output circuit tuned to select the same frequency band and coupled to said receiving apparatus, and a com mon resistance shunting each of said circuits, the input impedance of said device being adapted to match the terminal impedance of said line and the output impedance thereof being adapted to match the input impedance 01 said receiving apparatus.

2. In a carrier telegraph system, a line, signal receiving apparatus, an attenuation device for attenuating without distortion signals received over said line, said device having an nput circuit tuned to select a predetermined frequency band and coupled to said line and an output circuit tuned to select the same frequency band and coupled to said receiving apparatus, and a common and variable resistance shunting each of said circuits, the input impedance of said device being adapted to match the terminal impedance oi said line and the output impedance thereof being adapted to match the input impedance of said receiving apparatus.

3. In a carrier telegraph system, a line, signal receiving apparatus, an attenuation device having a balanced input circuit tuned to pass a predetermined frequency band and coupled to said line and a balanced output circuit tuned to pass the same frequency band and coupled to said receiving apparatus, the input impedance of said device bein adapted to match the terminal impedance of said line and the output impedance of said device being adapted to match the input impedance of said receiving apparatus, and a variable resistance. coupling said circuits together, said device being adapted to attenuate signals transmitted on frequencies within said predetermined band with the same amount of impedance throughout said frequency band.

ROTHWELL E. SMITH. LEO A. KELLEY. 

